


Happy Birthday

by Music_Junkie



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, Birthday Sex, Ecto-Genitalia (Undertale), Fluff, Gen, Incest, M/M, Oral Sex, Semi-Public Sex, Skeleton Sex, a little bit of smut
Language: English
Status: Completed
Published: 2016-09-26
Updated: 2016-09-26
Packaged: 2018-08-17 11:43:15
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,508
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8142503
Author URL: https://archiveofourown.org/users/Music_Junkie/pseuds/Music_Junkie
Summary: Fluffy Underswap fontcest. Underswap Sans gets spoiled silly on his birthday by his brother, who is more than happy to do so.





	

**Author's Note:**

> So! Here goes my first shot at Swapcest! this is for Buttercupsticksnlicks over on tumblr, as a belated b-day gift. She loves the Underswap bros, so I said I'd write them for her. 
> 
> (Man, I need to start checking when some of these birthdays are so I can actually do something ON TIME for them. Grrrr.) Regardless, I hope you like this, hun! And once again, my lack of naming skills shows. This is what happens when it's all I can think of. Ugh.

 

To say that Sans’ birthday started like any other day would be mostly correct. He’d slept with his brother, and gotten used as a teddy bear, because Papyrus had a tendency to cling.  
  
(Papyrus was a freaking _octopus_ at night, actually, but it made Sans feel safe, and as an added bonus, Pap was always warm.)  
  
One of the main differences was that this was his first birthday on the surface. Granted, he’d had a party and such with his friends the day before, but the actual day was a different story.  
  
He had the day off, so did his brother, and they’d elected to simply spend the day together.  
  
Sans was confused when he woke up, and Papyrus wasn’t there. The sound of swearing from the kitchen, however, alerted him to where his brother had disappeared to, and he grinned into the pillow.  
  
He debated on getting up, but he was really comfortable, and there was no smell of smoke, and well...it _was_ his birthday, he was entitled to a little relaxation. Plus, it was past morning, and heading into early afternoon anyway.  
  
Except that laying around just wasn’t happening. With a sigh, he rolled out of bed, just as Papyrus entered with a tray.  
  
“aww, you’re up already. I was gonna surprise you with breakfast in bed.”  
  
“I can get back in bed?” Sans promptly dived back into the bed and pulled the blankets over himself. “Better?”  
  
“guess so. y’ didn’t have to though. anyway, breakfast. and it came out decent for a change!” Papyrus settled the tray over Sans’ lap and sat at the end of the bed, watching expectantly.  
  
Sans poked at it curiously. It certainly _looked_ alright, but. “...it’s not gonna come to life, is it?”  
  
“ONE TIME. _ONE TIME,_ AND YOU NEVER LET ME FORGET IT.”  
  
“I’M SORRY, BUT WHEN YOUR OATMEAL RISES OUT OF THE BOWL, IT LEAVES AN IMPRESSION!”  
  
“I SAID I WAS SORRY!” Papyrus huffed. “besides, it’s just eggs and toast and juice, no oatmeal involved.”  
  
“Do I ask why?”  
  
“it may have stuck to the pot.” At Sans’ dubious look, he raised his hands in defense. “it didn't come to life, and I'm letting the pan soak, don't look at me like that, eat your breakfast. well, breakfast as lunch, anyway.”  


Sans tentatively took a bite of the scrambled eggs, humming in delight. The toast was next, with a thin layer of honey butter over the top. Even the orange juice tasted fine.

“Okay, how’d you do it?”

Papyrus looked innocent. “do what?”

“You made breakfast, you didn't burn it, you didn't forget about it, and the only thing that was ruined was the oatmeal. So how bad is my kitchen?”

“ _your_ kitchen? funny, thought we shared this place.”

“You know what I mean.” A bright blue stare was fixed on Papyrus, who stared back.

There was a stare-off for a moment before Papyrus looked away, rubbing the back of his skull. “the kitchen is fine. the first batch of toast burned because I had the toaster set too high, and I was aiming for soft fried eggs, but the yolks busted so they became scrambled, and we're out of sausage and peppers or I would've them put it in with the eggs.”  
  
“YOU WOULD’VE MADE ME A BREAKFAST SCRAMBLE?! AWW, PAPY!” Sans carefully set aside the breakfast tray, pushed back the blankets, and tackled his brother. “THAT’S SO NICE OF YOU!”  
  
“OOOF.” Papyrus readjusted Sans so he was straddling his lap and not pressing pointy elbows and kneecaps into his legs.  
  
“THANK YOU! YOU’RE THE BEST!”  
  
“ uh-huh. don’t think I should’ve if you’re gonna make fun of my cooking.”  
  
Sans pressed his teeth against Papyrus’ skull. “I love you~”  
  
“nope.”  
  
Kiss.  
  
“still no.”

Kiss.  
  
“uh-uh, buttering me up is not working.”  
  
Kiss.  
  
“....okay maybe a little.”  
  
Several more kisses.  
  
“alright fine, I love you too. happy birthday, bro.” 

“Thank you! It’s off to a good start! By the way, what should I expect for today? I’m off, you’re off, we’re both off for the next two days, we get to hang out together.”  
  
“I dunno about tomorrow, but we’ve got plans for today, and I know you’re gonna be cooking, ‘cause you don’t like my experiments at dinner. ”  
  
Sans pulled back and gave his brother a flat stare. “Two words: Spaghetti. Tacos.”

“alright, but that would’ve been a good idea if I’d _baked_ the spaghetti. and if I’d used soft shells instead of the hard ones.”

“And if you’d left the sour cream off.”  
  
“look, the cheese helped. next time, soft shells, no sour cream, and bake the spaghetti.”  
  
“Who says there’s gonna be a next time?”  
  
“okay, but it’s a good idea, you have to admit.”  
  
Sans rolled his eyelights. “I admit to NOTHING.”  
  
Papyrus stood up, still holding his brother. “will you admit that I have good ideas sometimes?”  
  
“Sometimes, yes.” Sans looped an arm around Papyrus’ neck, the motion familiar as always.  
  
“then you’re gonna love this.” Papyrus walked to his bedroom, and set Sans down on the bed. “go ahead and take your pjs off, I got something for you to wear. promise, I won’t look.”  
  
“You’ve seen everything I have anyway.” Sans promptly tugged his night shirt off, and swatted his brother’s hands away as he went for the newly exposed ribcage.  
  
“and then some.” Papyrus winked, and disappeared into his closet.  
  
“YOU’RE AWFUL!” Sans’ cheekbones turned bright blue.  
  
“eh, you love me and you know it. close those eyesockets.”  
  
Sans obediently did as he was instructed, despite the rustle of clothing raising his curiosity. After a moment, his arms were raised and guided through something cool, then lowered.  
  
“hold out your feet.” Socks were carefully rolled over small phalanges, and Sans giggled as Papyrus snuck in a quick tickle.  
  
“almost done. lay back and raise your hips for me.”  
  
Sans flopped back, spreading his arms. “Can I open my eyes yet?”  
  
“in a little bit.” Papyrus slid the shorts over Sans’ legs and hips, tapped his skull lightly, and stepped back. “okay, I’m all done with you. don’t open your eyes yet though. sit up if you want, I just gotta change real quick.”  
  
Barely five minutes after that, Sans was picked up again, then set down. “okay, open your eyes.”

Sans did so, and stared at the mirror they were standing in front of. From head to toe, he and his brother were decked out in baseball jerseys, shorts, and striped knee socks. His grin got wider. “WE MATCH!” He bounced excitedly. “PAPY, WE MATCH!”

“yep. figure we should, since we’re going to a baseball game.”  
  
Sans turned around and leaped at his brother, who caught him reflexively.  
  
“THAT’S SO AWESOME! WHO ARE WE GONNA SEE?”  
  
“we’re gonna go see Chara play, their team is facing off against another local one. Asgore invited us the other day, remember?”  
  
“I THOUGHT THAT WAS TOMORROW!”  
  
“nope, it’s today, that’s why I got us baseball gear. and would you please, for the love of everything good, try not to egg the kid on too much? they’re already competitive.”  
  
“COMPETITION IS A GOOD THING! CHARA AND THEIR TEAM WILL _CRUSH_ THE OTHER TEAM!” Sans pumped his fists in the air, throwing himself backwards and nearly getting dropped in the process.  
  
“that’s it, I’m putting you down. and you can go sit by Alphys too, when we get there.”

“I’M SITTING BY YOU.”  
  
"you’ll probably end up _on_ me, considering how wound up you are already. now go get your shoes on.”

Sans scurried off, then stopped and turned back. “Hey, what are we doing tomorrow? Should I not get my jersey dirty?”  
  
“well, it doesn’t matter if you do or not, these are just for solidarity. and I don’t have anything planned for tomorrow, thought I’d let you pick.”  
  
“NEAT!”  
  
The drive to the baseball field was short, and Sans tumbled out of the truck as soon as Papyrus turned it off, looking for a certain human.  
  
He didn’t have to look very long, as said human was running towards him and scooping him up into a tight hug.  
  
“HI SANS! HAPPY BIRTHDAY!”  
  
“HI CHARA! THANK YOU!”  
  
“HI PAPYRUS!”  
  
“hey kid.” Papyrus caught Chara as they launched themselves toward him. “are ya ready for your game?”  
  
“Yep! Dad and Toriel are finding seats, an’ I made sure everyone was invited, but it took a lot of convincin’ to get Undyne out of her lab.”  
  
Papyrus snickered. “did you sic Alphys on her?”  
  
“Yes. Desperate times, desperate measures, blah blah blah. Are you guys gonna be sitting with everyone else?”  
  
“depends on where everyone else is sitting.”  
  
“I can show you.”  
  
Papyrus made to set Chara down, only for them promptly pull their legs up and refuse to touch the ground.  
  
“....seriously kid, you expect me to pack you over to the field?”  
  
“Please with a cherry on top? You don’t want me to wear myself out before the game, do you?”  
  
“They have a point, Pap!”  
  
“See, Sans agrees with me!”  
  
“I’m so done with both of you.”  
  
Sans laughed. “You say it, but you don’t mean it.”  
  
“no I don’t. alright, you tiny menace to my peace of mind, point the way.”  
  
“TINY?!”  
  
“you heard me.” Papyrus started walking, Sans beside him. “though that applies to Sans too, when you consider some of the stuff he does.” He dodged a kick from his brother, snickering.  
  
“I’M NOT TINY! I’M….VERTICALLY CHALLENGED!”  
  
“you keep telling yourself that, bro.”  
  
After the trio reached the bleachers, Chara was handed off to Asgore, who set them down and walked them to the field.  
  
Sans immediately plopped himself down next to Toriel and Alphys, making a face at Papyrus, who rolled his eye lights and sat next to Undyne.  
  
By the time the game was half over, both Sans and Papyrus’ predictions had come true, in that Chara’s team _was_ beating the other one (by a slim margin of 3 points), and Sans had ended up in his brother’s lap, bouncing and squirming and cheering every time Chara or a familiar monster child was up to bat.  
  
Several times, Papyrus had to grab the back of the other skeleton’s shirt to keep him from toppling off his legs whenever he leaned forward too far.  
  
When the game ended, Sans slumped against Papyrus.  
  
“THAT WAS FUN, PAP.”  
  
“glad you had a good time.” Papyrus patted Sans’ sternum. “now get off me.”  
  
“Nooooo.”  
  
“yeeeees.”  
  
“Noooo, I don’t wanna.”  
  
“hey alphys, come get your trainee.”  
  
“Nah, I think I’ll leave him in your hands.”  
  
Sans gave her a thumbs up. “Thanks Alphys!”  
  
"fine. undyne, don’t you need an anime partner?”  
  
“QUIT TRYING TO GET RID OF ME!”  
  
“I, uh...I already h-have one, Alphys is coming over, we’re, um, going to watch the second s-season of Cardcaptor Sakura.”

Papyrus shifted, sitting up straighter. “oh hey, lemme know what happens? I heard there’s a third season too.”

“Sure! I’ll leave out the spoilers on important stuff, but I-I can give you a rundown on what else happens.”

“cool. lady toriel, I don’t suppose you’d want him?”  
  
Toriel laughed. “I believe I’ll pass, but thank you.”  
  
“well, it was worth a shot.”  
  
Sans gave Papyrus a dirty look before he turned himself around to cling to him koala-style, sulking. “Not getting rid of me.”  
  
“yep, looks like we’re stuck with each other, but I had to try.” Papyrus chuckled and rubbed Sans’ spine as the other thumped a small fist against his scapula. “you know I love you, I’m just kidding.”  
  
“Uh-huh.”  
  
After more small talk, Chara was rounded up to say goodbye, and they wiggled a promise from the skeleton brothers to play pinball at the arcade.

Soon, the bleachers and the field were empty except for two skeletons who had ducked under the bleachers when they got a chance.

Sans clung to his brother the whole time. “Dare I ask what we’re doing down here?”  
  
“you mean beside that you got yourself worked up on purpose? bouncing around and grinding on me? you have been all over me today.”  
  
Sans hid the very smug look that had appeared on his face in Papyrus’ shoulder. After all, his brother _was_ right, he _had_ been doing it on purpose. “You say that like it’s a bad thing.”  
  
“ brat.” A quick kiss to the top of Sans’ head, and Papyrus raised his brother up just enough that the smaller skeleton could hold onto the bench above his head.  
  
“hang onto that for me? like a good boy?”  
  
Sans nodded, grabbing the bars underneath the bench.  
  
“thank you. you’ve been driving me crazy all day, you know that?”  
  
“Your fault for buying me the outfit,” Sans protested halfheartedly. His shorts were tugged down to his ankles, which Papyrus had rested on his shoulders.

“hmm, maybe. or maybe I just underestimated how good you look in these shorts.” Papyrus leaned forward and ducked under the bunched up fabric to flick his summoned tongue against the ball of magic resting in Sans’ pelvic inlet. It formed into an actual pussy, and Papyrus licked again.  
  
_“Pap!”_  
  
“uh-uh. hold on. we’re just getting started.”  
  
“S-someone might - _hnh_ , see!”  
  
“doubtful. ‘s just you and me, bro. everyone else is gone. but if you wanna stop, we can.”  
  
“N-no, I’m-” a sloppy lick made his voice rise in pitch. ”OKA _AAYYY_!”    
  
With that, Papyrus buried his skull between Sans’ femurs. It didn’t take much to get Sans moaning, since he was already pretty sensitive, but he was still holding on. Clearly, Papyrus would have to try harder. Which he did, and pretty soon Sans was clutching at his brother’s skull with one hand.  
  
“Pa-Papy, _p-please,_ I….ah! Nnn, y-you’re not-hah, playing f _-faaaaair!_ ”

Papyrus hummed loudly, the vibrations making Sans’ femurs shake as he came. A quick check showed that Sans’ eye lights had gone heart-shaped, expression dazed and happy. His soul was glowing brightly through his shirt, practically illuminating the dark space.  
  
Papyrus kissed the inside of San’s femurs, pleased. It took a moment for Sans to focus again, during which his brother had wiped his face off, pulled his shorts up and adjusted his grip so that with a gentle tug, Sans let go of the bleachers and slumped over him instead.

“okay there?”

“Uh-huh.”  
  
“good. shall we head home?”

“Yeah.”  
  
“what do you want for dinner?”  
  
“Don’t care.”  
  
“goofball. you got yourself all worked up and worn out.”  

“HALF OF THAT IS YOUR FAULT!”  
  
“yeah yeah.”  
  
They stopped for burgers and fries for dinner, followed by frozen custard. By the time they got home, Sans had perked back up, and insisted on couch cuddles and a movie, claiming it’d be a perfect end to a perfect birthday. Papyrus was more than happy to oblige.  
  
A quick shower and change into pajamas, and a search for their respective movie night blankets, and the brothers settled in to watch _Mission Impossible: Rogue Nation._  
  
The next morning found them both stretched out on the couch, snoring in two part harmony.


End file.
